I'm totally faint in using multer-s3 to upload image to aws s3 bucket

what does "upload.array('photos', 3) " in the following code denotes ....

My html code. (code reduced for brevity)

 <form action="/upload" method="POST">
 <input type="file" name="pic" id="pic" accept="image/*">
 <input type="submit">
 </form>

my expressjs code

  AWS.config.update({
  accessKeyId: "xxxxxxxxxxxxx",
  secretAccessKey: "yyyyyyyyyy", 
  "region": "zzzzzzzzz" ,
  signatureVersion: 'v4'
  });

var s3=new AWS.S3();

var upload = multer({
storage: multerS3({
s3: s3,
bucket: 'imgcontainer',
metadata: function (req, file, cb) {
  cb(null, {fieldName: file.fieldname});
},
key: function (req, file, cb) {
  cb(null, Date.now().toString())
 }
 })
})

 app.post('/upload', upload.array('photos', 3), function(req, res, next) {
    res.send('Successfully uploaded ' + req.files.length + ' files!')
   })

output:

 Cannot read property 'length' of undefined

via V Abinaya